Method of making terminal bushings



J. H. M WHIRTER METHOD OF MAKING TERMINAL BUSHINGS 4 Sheets-Sheet 1 Filed Jan. 11. 1961 AHQ GHQ INVENTOR James H. McWhirter ATTORNEY J. H. M WHIRTER METHOD OF MAKING TERMINAL BUSHINGS Aug. 9, 1966 4 Sheets-Sheet 2 Filed Jan. 11. 1961 Fig.4

Fig. 6

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Aug. 9, 1966 J. H. MCWHIRTER METHOD OF MAKING TERMINAL Busmms Filed Jan. 11. 1961 4 Sheets-Sheet 3 77 Fig. 7

Aug. 9, 1966 J. H. MCWHIRTER METHOD OF MAKING TERMINAL BUSHINGS 4 Sheets-$heet 4 Filed Jan. L1, 1961 mm mm mk h nob E om 7 I w m United States Patent 'METHOD OF MAKIN TERMINAL BUSHINGS James H. McWhirter, Sharon, Pa., assignor to Westinghouse ElectrieCorporation, East Pittsburgh, Pa., a corporation of Pennsylvania Filed Jan. 11, 1961, Ser. No. 82,025 4 Claims. (Cl. 264-262) a This invention relates to a method of making terminal bushings in general, and, more particularly, to a method of making terminal bushings, which are cast from a synthetic, thermosetting resin and to improved methods of constructing them.

The method of this terminal bushing, in which internal stresses are maintained at a minimum by employing a plurality of precast component parts.

At the present time, the electrical industry employs large numbers of terminal bushings in generators, transformers, switchgear and similar apparatus. For highvoltage use, the terminal bushings are usually constructed of a central conducting stud and an exteriorly-spaced porcelain sleeve sealed to the ends of the stud. The space between the porcelainsleeve and the conducting stud is filled with oil in order to provide dielectric strength, and to dissipate heat developed by the passage of electrical current through the stud. In many cases, the space between the porcelain sleeve and the centrally-disposed terminal stud is partially filled with a wrapping of paper, but oil is necessary to insure proper operation of the terminal-bushings in service. The oil filling enables terminal bushings to meet certain standards of impulse strength and to withstand surges. Furthermore, it is necessary that terminal bushings do not generate radio-interfering energy. I i i For higher-voltage service, metallic foil layers may be interspersed within the layers of paper to provide condenser elements for grading'the impressed voltage between the oentrally-disposed conductor stud and the outer grounded flange sleeve. As' the output of generators and transformers has increased, particularly with considerable increases in am perage, the size of oil-filled terminal bushings must be increased correspondingly. The problem of providing an oil-tight seal between the ends of the porcelain sleeve and the centrally-disposed terminal stud becomes increasingly more diflicult, as the size of the terminal bushing increases. This is due to the fact that during firing, porcelain shrinks non-uniformly, and erratic changes in dimensions occur, such changes increasing with size. Other disadvantages, such as the fire hazard, cost and increased bulk of the apparatus, necessitated by the larger-size bushings, has made it less and less desirable to use an oil-filled terminal bushing. However, in spite of these disadvantages, oil-filled terminal bushings have been extensively utilized in the art. F V

In an effort to overcome some of the obvious aforementioned disadvantages of oil-filled terminal bushings, it has been proposed to provide terminal bushings cast of epoxy resin. Both domestic and foreign manufacturers have, for some time, considered such a cast-epoxy-resintype of terminal bushing. The method most often advanced has been to cast the terminal bushing in a single castingfrom a synthetic thermosetting resin, so as to include the condenser, weathercasing and mounting flange. However, many difiiculties have been encountered as follows:

(1) The resin shrinks in hardening, whereas a metallic.

insert, which may be used for the condenser layer, does not shrink. The result is that stresses are set up in the H resin, which leads to cracking.

(2) The resin usually has a coefiicient of thermal expansion different frointhat of the metallic condenser invention provides an improved layer. This creates stresses, which add to the shrinkage stresses at low temperatures.

(3) The adhesion of resins to smooth metallic surfaces is poor relative to the adhesion of the "resin to itself. This creates a weak spot, which is likely to develop into a void with thermal cycling. Conducting paintfwhich has been tried as a condenser layer, also results in poor adhesion, which is likely to develop into a void.

. It is, therefore, a distinct object of the present; invention to provide a method of making an improved terminal bushing cast from a synthetic thermosetting resin, which will avoid the difiiculties set forth above.

Another object of the present invention is to provide a method of making an improved terminal bushing formed from pre-cast parts to minimize the problem of shrink ing,,which has heretofore confronted the manufacturers of terminal bushings.

Another object of the present invention is to employ improved centrifugal-casting techniques in theprovision of animproved terminal bushing formed from a synthetic thermosetting resin.

Still a further object of the present invention is to provide improved methods for the centrifugal casting' of terminal bushings from a synthetic thermosetting resin.

Yet a further object of the present invention is to provide a method of making an improved terminal bushing of the condenser-type, in which conducting screening is employed in a novel manner with a plurality of pre-cast component parts of the terminal bushing.

Further objects and advantages will readily become apparent upon reading the following specification, taken in conjunction with the drawings, in which:

FIGURE 1 is a fragmentary diagrammatic view of a centrifugal-casting apparatus, which may be employed in the construction of an improved terminal bushing in accordance with the teachings of the present invention; FIG. 2 is a perspective view of one of the component mold parts, which may be employed in connection with the centrifugal-casting apparatus of FIG. I, with one of the perforated end plates removed to show certain features of the invention;

FIG. 3 is a sectional view taken through an oven to cure the resin while in the molds; 7

FIG. 4 is a perspective view of a plurality of the precast parts, which result from an employment of the centrifugal-casting apparatus of the type illustrated in FIGS. 1 and 2, showing, in addition, their relationship with each other;

FIG. 5 illustrates, in end elevation and fragmentary side elevation, the manner of wrapping conducting screening about a pre-cast resin cylinder, and utilizing glass string to maintain the same in proper position;

FIG. 6 illustrates, in plan, a screening section used for one of the condenser layers;

FIG. 7 illustrates, in vertical cross section, the method employed to admit resin to the space between the'successive tubes to obtain the complete casting of the terminal bushing;

FIG. 8 illustrates, in fragmentary manner, the final machining, by a cutting tool, of the completed cast-body portion of the terminal bushing;

FIG. 9 illustrates in vertical cross-section an entire completed terminal bushing of the type proposed in the present invention, subsequent to the machining operation;

FIG. 10 illustrates a modified type of centrifugal-casting technique, and the rotatable casting apparatus which may be employed, to provide an improved condenser-type cast-resin terminal bushing in accordance with further teachings of the present invention;

FIG. 11' illustrates the final step of casting the terminal stud and its attached screen within the hole left in the terminal bushing following the step of FIG. 9; and,

FIG. 12 shows, in side elevation, the finished terminal bushing according to either method. 1

Referring to the drawings, and more particularly t FIG. 1 thereof, the reference numeral 1 generally designates a centrifugal-casting apparatus, which is preferably used in accordance with the present invention. It will be noted that a drive-shaft or mandrel 2 has secured thereto a drive-wheel 3, which is driven by a belt 4 from the pulley 5 of a suitable motor 6.

The drive-shaft 2 is mounted in block bearings 7, 8 which may be split, as shown, to permit their ready removal. The bearings 7, 8 are preferably mounted upon a pair of channel-shaped supporting members 9 mounted upon a suitable base or steel frame 10.

FIG. 2 shows a typical split mold 11 comprising two portions 12, 13, which may be bolted together, as at 14. Preferably the mold 11 is additionally provided with a pair of end plates 15, which have a hub portion 16 to facilitate attachment to the rotatable drive shaft 2. A gateopening a may be provided in one end plate 15, as shown. The end plates 15 may be secured to the drive shaft or mandrel 2, so as to rotate therewith, by any suitable means, for instance as by a press fit.

Bolts17 may be employed to fixedly secure the end plates 15 to the opposite ends'ofthe splitmold 11.

Asshown in FIG.,1, a suitable container 18 may be employed, having a spout 19 to admit a measured quantity of a suitable synthetic resin, more fully described hereinafter, to the interior region 20 (FIG. 2) of the mold 11. Heat lamps 21 are provided, as shown, to heat the molds. During the spinning of the centrifugal-casting apparatus 1, the synthetic resin will form a uniformly thin layer overfthe entire inner surface of the mold 11.

For example, such a thin coating may be substantially inch thick. The motor 6 may run at a speed of say 200 r.p.m.

By rotating the mold 6 for approximately 1 hour, the synthetic resin will gel, and when this occurs, the several molds 11, 11a, 11b 14n maybe removed from the centrifugal apparatus 1,. and placed within. an oven 22, more clearly shown in FIG. 3 of the drawings. The time and temperature required for gelling of the resin can be controlled within wide limits by the use of inhibitors and accelerators in the resin mixture.

In the oven 22, the temperature may be regulated by heaters 23, so that the synthetic resin may have a curing time of, say' 16 hours. When this occurs, the resin will have a hard tough characteristic.

Following removal from the oven 22, the molds 11, 11a, 11b, etc. may be split by removal of the clamping bolts 14. For certain applications, the molds need not be split, but merely tapered to facilitate removal of the cured pre-cast cylinders.

With reference to FIG. 4 of the drawings, it will be observed that a plurality of pre-cast sleeves 25, 26, and 27 may be formed by the aforesaid centrifugal apparatus. Preferably each sleeve 25, 26, 27 has an external diammaintain the screening 28 in proper position. ,a suitable tween the adjacent insulating layers is essentially resin-toresin through holes in the screen, rather than being de-' The screening 28 is then wrapped around the roughened cylinder 27, as indicated in FIG. 5 of the drawings. To

glass string 24 may be employed as shown in FIG. 5. g

If desired, the screening could be applied in tape form rather than as a solid sheet, as shown.

The use of metallic screening within a resin-cast terminal bushing is described and claimedin United States patent application filed July 23, 1959, Serial No. 829,091 by C. F. Sonnenberg, now Patent No. 3,001,005 and assigned to the assignee of the instant application.

A resin-cast outer supporting flanges 24 may be ccnseveral pre-cast cylinders 25, 26, 27, a suitably-formed clamping nut 31 may be screwed upon the lower threaded extremity 32 of a conducting stud 33, as indicated in FIG. 7 of the drawings. The stud may have an innermost screen wrapped thereabout and electrically attached thereto by a conducting strap 77, as shown. This will prevent leakage of the liquid resin out of the lower end of the terminal bushing 34 between the nested tubes 25, 26, 27, etc. To assist in admitting the resin 35, a suitably-configured guide-cylinder 36 may be employed. 7

The problem of matching the coefiicient of expansion of the cast resin to that of the copper screenand central stud is solved by the use of a beryl filler material. A fine powder beryl has a very low coefiicient of thermal expansion, and when added to the resin has the effect of reducing the effective coefficient of the mixture. Thus, the two materials will expand and contract together with changes in temperature. It has also been observed that advantages, but because of the small spaces 37 it may eter only slightly smaller than the internal diameter of the immediately adjacently disposed outer sleeve. This is somewhat indicated, in diagrammatic form, in FIG. 3 of the drawings by the nested arrangement.

It has been found that if pre-cast parts are used, the adherence to, the pre-cast part of a subsequent casting is very good if the surfaces of the pre-cast part are cleaned and roughened by sand blasting. The joint'is as strong as the rest of the casting. Consequently the cylinders 25, 26 and 27 may be sand-blasted prior to subsequent assembly steps.

Subsequent to the casting of successive tubes, or shells 25, 26 and 27, conducting screening 28 is cut to the desired shape, as indicated in FIG. 6. The metallic screening is used to provide equipotential surfaces within the completed bushing. This is essentially equivalent to foil be possible to use an unfilled resin. With-an unfilled resin, it may be possible to do this without the use of vacuum, which would be highly desirable from a manufacturing standpoint.

FIG. 8 shows the completed terminal bushing 34 prior to the final machining operation. It will be-observed that the resin 35 fills the spaces 37 and results in the formation of an integral body 40, which may subsequently be machined along a tapered line 41 by a suitable cutting tool 42 mounted in a lathe, as well understood by those skilled in the art. The purpose of providing the machining operation is to provide a smooth external surface 29 to minimize the possibility of dirt and contaminating particles from collecting within the shoulder portions 43 of successive cylinder layers 25-27.

Any suitable resin, which is capable of beingcen desirable casting resin is provided by a completely reactive resinous composition such as admixtures of (1) 1 mol of a glycidyl polyether of a dihydric phenol, (2)

. 15 mol percent of the an olefinically unsaturated dicarboxylic acid anhydride in an amount sufficient to provide approximately a stoichiometric number of anhydride groups to react with the epoxy and hydroxyl groups in said glycidyl polyether,

(3) at least one liquid unsaturated reactive monomer hav-.

ing the group C=C there being employed at least 2 mols of said monomer for each and (4) a tertiary amine. While it is preferred to employ a stoichiometric "molar amount of the anhydride with respect to the epoxy and hydroxyl groups in the polyether, satisfactory results are obtained when the anhydride is employed in an amount within plus or minus stoichiometric amount. The tertiary amine should be employed in an amount within the range of from0.0l% to 5% by weight, based on the total weight of the composition.

Although the resinous composition set forth in the aforesaid patent application by Foster is a particularly advantageous one, nevertheless, it will be apparent to those skilled in'the art that other resinous compositions, suitable for centrifugal casting, may be employed.

It will be noted that the problem of shrinkage of the resin during curing is minimized by the use'of pre-oast parts where the shrinkage has already occurred in such a way as to result in a'stress-free casting. Also centrifugal casting techniques are bushing constructions because of the simplicity of the. terminal bushing and its taxial symmetry.

FIG. illustrates a modified cessivepre-cast layers 25, 26,27 in a'termirial bushing 34. With reference to FIG.'10 a w n he noted that centrifugal-casting apparatus 46 ispi'ovided. The apparatus 46 includesa rotatable, split-mold structure 47, 48 having an end-clamping portion 49, which maybe removed to insert successive screening layers 28, 28a, 28b, etc. An inlet opening 50, closed by plug 51 may be provided to admit a measured quantity of a suitable resinous composition 35 to the interior 52 of the centrifugal mold 46. A drivejshaft 53, having a drive-pulley 54 is rotated by 'a suitable belt 55 by the pulley 56 of a drive motor 57. A second block bearing 58 is provided for a guide shaft 59. having a clamping flange 60. Bolts 61, which are readily removable, are employed to clamp the clamping flange 60 to a flange 62 of' the portion 49. Additional bolts 63 are employed to detachably clamp the portions 46 and 49. i

In operation, a measured' quantity of resinous cornposition 35 is admitted through the port opening 50 into .the interior 52 of the centrifugal mold 46.. The motor 57 brings the mold 46 up tospeed, and this is continued until curing of the resin 35 is obtained. When this has been achieved, the guide shaft 59 is removed by opening the split bearing 58 and end portion 49 is removed. A flexible screen 28 of the desired length and width is then inserted into the left-hand open end of the mold .46. The several parts are again clamped into position, andthe motor 57 is again started. The spinning of the mold 46 causes the flexible screening 28 to spread out uniformly over the previously-cast resinous layer 70 by the developed centrifugal force of the spinning mold. An additional premeasured amount of resinous composition 35 is then admitted. through the inlet opening 50, and this will form a second layer 74 (FIG. 11), which is subsequently cured in the manner as described above. Operations are continued in like manner until the terminal bushing 34 is built up.

Following the centrifugally casting of the layers 70, 74, 25, 26, 27, etc., as shown in FIG. 10, the mold 46 is split by removal of the clamping bolts 75. The unfinished terminal bushing, having a bore therethrough is placed in an upright position,.as shown in FIG. 11. The conductor stud 33 has electrically attached thereto an inner screen 28, as by a strap 77. This assembly is inserted into the remaining bore 78 of theunfinished bushing 34. A stop sleeve 79 is screwed upon the threaded particularly applicable to i portion 32 of conductor stud 33 and resinous composimol of said anhydride,

method of providing suc- A tion 35 is poured into the upper end of between bore 78 and conductor stud 33.

Following curing of this innermost resinous layer, the stop cap 79 is removed and the terminal bushing 34 is now complete. FIG. 12 illustrates, in side elevation, the finished terminal bushing 34.

With the second method shown in FIG. 10, it will be noted that the machining step, set forth in FIG. 8, may be eliminated, since the inner wall 72 of thecentrifugal mold 46 may be tapered in the desired manner.

From the foregoing description of two ways of centrifugally casting a terminal bushing 34, it will be noted that internal stresses are minimized.

The present invention is not restricted only to the illustrated methods of forming a terminal bushing 34. The present invention also broadly encompasses the use of a plurality of pre-cast sleeves 25-27 (no matter how they are formed) in conjunction with the screening 28, 28a, 28b, and any method of introducing a resinous composition 35 between the spaces 37, as illustrated in FIG. 7.

A resin casting involvesa compromise between flexibility and good electrical and thermal stability properties.

annular space 80 With these techniques it is possible to use the harder better thermal stability. The problems of cracking will also be reduced by the use of aluminum screen rather than'copper. It is possible to formulate a filled resin which will match the .coeflicient of thermal expansion of the aluminum. This essentially eliminates the stresses of thermal cycling. Experience indicates'that the copper screening is satisfactory, however, other open type conducting materials such as a conducting cloth may be used also;

These methods should yield a smaller and lighter bushing with very few tcst rejections and a satisfactory service life.

Although there has been shown and described specific methods and structures, it is to be clearly understood that the same were merely for the purpose of illustration and that changes and modifications may readily be made therein by those skilled in the art without departing from the spirit and scope of the invention.

I claim as my invention:

1 In the process of preparing a unitary laminated terminal bushing of a resinous composition including a centrally-disposed conductor stud, the steps comprising forming a plurality of cooperable tubes of graduated diameters of a resinous material in a manner to provide tubes of substantially uniform wall thickness and of substantially uniform dielectric strength, heating the several tubes of resinous material to thermoset the tubes to a solid and shrink the tubes whereby further substantial shrink age occurs during the remaining steps of the process, wrapping conducting screening aboutseveral of said tubes, disposing the .tubes together concentrically, introducing into the annular spaces between the tubes :1 liquid thermosettable composition which flows through the mesh of the screening and adheres to the inner and outer walls of the tubes, and curing the last-mentioned composition.

2. In the process of preparing a unitary laminated terminal bushing of a resinous composition including a centrally-disposed conductor stud, the steps comprising forming a plurality of cooperable tubes of graduated diameters of a resinous material in a manner to provide tubes of substantially uniform wall thickness and substantially uniform dielectric strength, heating the several tubes of resinous material to thermoset the tubes to a solid the tubes a liquid therv .moscttable composition which flows through the mesh in the screening and adheres closely to the inner and outer wztlls of the tubes, and curing the last-mentioned composition.

3. In the process of preparing a unitary laminated terminal bushing of a resinous composition including a c'en trally-disposed conductor stud, the steps comprising forming a plurality of cooperable tubes of graduated diameters introducing into the annular spaces between the tubes a' liquid thermosettable composition which flows through the mesh in the screening and adheres to the inner and outer w'aIls of the tubes, and curing the last-mentioned composition. v

4. In the process of preparing a unitary laminated terminal bushing of a resinous composition including acentrally-disposed conductor stud, the steps comprising forming a plurality of cooperable tubes of graduated diameters of a resinous material to provide tubes of substantially uniform wall thickness and of substantially uniform dielectric strength, heating the several tubes of resinous material to thermoset the tubes to a solid and shrink the tubes whereby no further substantial shrinkage occurs during the remaining steps of the process, wrapping conducting screening about several of said tubes, wrapping glass string over the screening to secure the same in proper position, disposing the tubes together concentrically. introducing into the annular spaces between the tubes a liquid thermosettablc composition which flows freely through the mesh in the screening nndudheres to the inner and outer walls of the tubes, and curing the lust-mentioned composition.

References Cited by the Examiner UNITED STATES PATENTS 830,432 9/ 1906 Hennessey. 1,606,680 11/1926 Wisner. 1,834,212 12/1931 Jannson, 1,871,356 8/1932 Bakken 174-143 2,126,707 8/ 1938 Schmidt. 2,298,211 10/1942 Guzik 174-l43 2,349,549 5/1944 Hardman 1858.3 2,530,835 11/1950 Muller 1542.6 2,616,481 11/1952 Twells et al. 1859 2,731,068 1/1956 Richards 154-2.6 2,785,442 3/1957 Boggs l8-58.3 3,002,646 10/1961 Lewis 264248 FOREIGN PATENTS 1,144,430 4/ 1957 France.

742,413 12/1955 Great Britain. 772,054 4/1957 Great Britain.

ROBERT F. WHITE, Primary Examiner.

MICHAEL V. BRINDISI, WILLIAM STEPHENSON,

ALEXANDER H. BRODMERKEL, Examiners.

T. F. LAMONT, F. MARLOWE, L. S. SQUIRES,

Assistant Examiners. 

1. IN THE PROCESS OF PREPARING A UNITARY LAMINATED TERMINAL BUSHING OF A RESINOUS COMPOSITION INCLUDING A CENTRALLY-DISPOSED CONDUCTOR STUD, THE STEP COMPRISING FORMING A PLURALITY OF COOPERABLE TUBES OF GRANDUATED DIAMETERS OF A RESINOUS MATERIAL IN A MANNER TO PROVIDE TUBES OF SUBSTANTIALLY UNIFORM WALL THICKNESS AND SUBSTANTIALLY UNIFORM DIELECTRIC STERENTH, HEATING THE SEVERAL TUBES OF RESINOUS MATERIAL TO THERMOSET THE TUBES TO A SOLID AND SHRINK THE TUBES WHEREBY THE FURTHER SUBSTANTIAL SHRINKAGE OCCURS DURING THE REMAINING STEPS OF THE PROCESS, WRAPPING CONDUCTING SCREENING ABOUT SEVERAL OF SAID TUBES, DISPOSING THE TUBES TOGETHER CONCENTRICALLY, INTRODUCING INTO THE ANNULAR SPACES BETWEEN THE TUBES A LIQUID THERMOSETTABLE COMPOSITION WHICH FLOWS THROUGH THE MESH OF THE SCREENING AND ADHERES TO THE INNER AND OUTER WALLS OF THE TUBES, AND CURING THE LAST-MENTIONED COMPOSITION. 